Along the 400-kilometre route that cuts through Syria’s fractious war-ravaged north, they pass through numerous checkpoints manned by Kurdish and rebel factions.

At each checkpoint they must pay a toll and by the time they reach Maarat Al Naasan, they will have paid the equivalent of $17 per barrel in fees.

Abu Al Omarain said he and fellow truck drivers used to bring crude from fields controlled by Daesh in Syria’s oil-rich eastern Deir Al Zor province, but not anymore.

The militants now hold just a sliver of the province after losing out to twin offensives by Russian-backed regime forces and US-supported fighters.

On a recent day Abu Al Omarein sat on the ground at the entrance to Maarat Al Naasan, sipping a warm cup of tea and chatting with fellow drivers as the crude oil barrels were unloaded from the trucks.

The crude is funnelled into a large metal cylinder where it is heated and separated into gasoline, kerosene and diesel.

The liquids then pass along metal pipes through a basin of water, which cools them down so they can be collected in plastic containers for sale.

The processing costs $24, leaving consumers paying an average of $95 per barrel.

Malek Haj Hamdan says he relies on these primitively refined oil products to run his bakery because they are more affordable than those provided, irregularly, by the Syrian government.

The oil refined in Maarat Al Naasan is 30 per cent cheaper than what arrives from regime areas, he says, adding that this ultimately means that he can sell his bread at a lower cost.

But this oil also contains many “impurities” that damage his baking equipment, says Hamdan.

The rudimentary refining process has also taken a toll on the health of residents, according to Fahed Al Abd, a specialist in internal and cardiac medicine at Maarat Al Naasan’s medical centre.

Around 2,000 patients a year, mostly children under 15, suffer from complications from “the organic materials spread in the air as a result of the refineries and the primitive processing of crude petroleum products,” says Abd.

Six-year-old Abdul Nasser was one of those affected in Maarat Al Naasan.

Around a year ago, dark spots began appearing in a thick diagonal line across his cheeks, his father Abu Yahya says.

“There were a lot of refineries around town putting out a tonne of smoke and gases and causing uncontrollable damage,” he says. “The biggest proof of this is my son.”

“Thank goodness, we are treating him now, but the doctor told me that the marks will be there for the rest of his life.”

The medical centre blames “gases and unclean air” emanating from the refineries for Abdel Nasser’s condition.

To scale back the environmental damage and adverse health effects, rebel authorities have set up strict schedules during which refineries can operate.

“In the past, the 100 refineries in Maarat Al Nasaan used to operate every day, all at the same time,” says refinery owner Riad Haydar, 37.